The present invention relates to a method of improving the CAS tone signal detection of a telephone which detects a small segment of the received CAS tone signal (CPE alert signal, in which CPE is an abbreviation of customer premises equipment), then mutes the voice signal from the near end subscriber, and then detects the time series of the left CAS tone signal and monitors the voice signal from the far end subscriber, so as to accurately measure the length of the CAS tone signal when no voice signal from the far end subscriber or the near end subscriber is detected, and then to determine, subject to the definition of the predetermined CAS tone signal length specification, whether the received CAS tone signal is true or false.
In addition to the conventional voice communication function, advanced telephones provide a digital communication function. CIDCW (Caller ID Call Waiting) identification function is the most popularly employed digital communication function. Through the exchange line from the telecommunication office, the telephone number, name and other information of the caller are transmitted to the telephone of the called party, and shown on display means for example a LCD (liquid crystal display) on the called party's telephone. Before receiving the telephone call, the called party can know the caller's identification through the display on the telephone.
Technically, a telephone with CIDCW identification function receives a CAS tone signal before receiving a CIDCW data from the exchange line of the telecommunication office. The CAS tone signal is to call the called party's telephone. Following the CAS tone signal to the called party's telephone is a CIDCW data. Therefore, preventing an interference from the near end subscriber and the far end subscriber so as to let the called party's telephone accurately receive the CAS tone signal upon the detection of the CAS tone signal is a prerequisite to the completion of the receipt of the CIDCW data. However, it is difficult to accurately receives the CAS tone signal.
According to British CIDCW specifications SIN 242 and SIN 227, the CAS tone signal has parameter characteristics as shown in FIG. 1. Because the frequency of the CAS tone signal is within the range of AF (audio frequency), the voice signal from telephone at the near end as well as the far end and external noises may be falsely detected as a CAS tone signal, causing the actual CAS tone signal unable to be accurately detected. For example, if the CAS tone signal is interrupted due to a voice signal or external noise interference, the tone signal is judged non-effective, and the following CIDCW data is rejected. This is the so-called talk-down performance in CAS tone signal detection. Further, if the caller's telephone does not transmit the CAS tone signal, and the called party's telephone make a wrong judgment due to the interference of a voice signal or external noises, the called party's telephone works falsely. This is the so-called talk-off performance in CAS tone signal detection.
A telephone with CIDCW identification function, as shown in FIG. 2, generally comprises a CAS tone signal detection chip (it can be a Mitel 8843 chip) 11, a talk circuit 12, a CPU 13, a microphone 14, a speaker 15, a bridge 16, a select/release circuit 17, and a LCD 18. The select/release circuit 17 receives the digital signal from the exchange line through signal terminals A;B, then transmits the received digital signal to the CAS tone signal detection chip 11 for detecting the presence of a CAS tone signal. Upon detection of the CAS tone signal, the CAS tone signal detection chip 11 immediately triggers the CPU 13, causing it to receive the following CIDCW data, and to show the caller's telephone number and name and other information through the LCD 18, and the voice signals from the caller and the called party are sent through the bridge 16 and the talk circuit 12 to the microphone 14 or the speaker 15.
Using the CAS tone signal detection chip 11 for detecting the presence of a CAS tone signal provides the advantages of low cost and ease of application. However, the talk-down and talk-off performance in CAS tone signal detection of the CAS tone signal detection chip 11 is low because the CAS tone signal cannot eliminate the interference of a voice signal or external noises. Therefore, this CAS tone signal detection chip 11 cannot meet US Bellcore's SR-TSV-002476 and British BT specification requirements. Further, a DSP (digital signal process) may be used for detecting the presence of a CAS tone signal. The DSP achieves high talk-down and talk-off performance, however it is expensive, and its application is difficult to be achieved.